1. Field of the Invention
This invention relates to a bobbin holding means (hereafter referred to as bobbin basket) for use in a rotary loop taker of a lockstitch sewing machine, and specifically to those bobbin baskets that require a bobbin case to retain the bobbin basket from axial movement.
2. Background Art
In further testing of products related to applicant's previous U.S. Pat. No. 4,858,543, dated Aug. 22, 1989, the applicant of the present invention unexpectedly discovered other structural weaknesses which either prohibited use, or severely limited the useful life, of the polymeric bobbin basket for use in a rotary loop taker. In many test cases, the center post head separated from the center post neck or the center post neck separated from the center post stem. The bobbin case moved from its stationary latched-in position and received the full impact of the needle at the needle entry hole causing fracture of the bobbin basket to occur.
Another problem occurs when thread accidentally becomes wedged between the bobbin case lever and the bobbin case locking plate as the needle thread loop passes over and around the bobbin basket as the rotary loop taker rotates. The thread loop tightens around the thin center post neck which severely stresses the center post neck in the direction of the thread loop take up. The lever then pivots from its fulcrum to a partially unlatched position which allows subsequent thread loops to wedge themselves under the bobbin case lever and tighten around the center post neck. Inevitably the center post head fractures at the center post neck or the center post neck fractures at the center post stem.
Another reason for the fracturing of the center post head from the center post neck is the relatively low modulus of polymeric materials relative to the metal materials used in the conventional steel loop taker. A material can endure a finite number of stress cycles. It is a well known fact in material science that metals can endure higher stresses than polymers in like configuration. In other words the fatigue resistance of polymers is lower than that of metals. At higher operating speeds, where vibrational frequency is greatest, the rotary loop taker endures a greater number of stress cycles at higher amplitudes. At lower operating speeds the stress cycles are fewer with lower amplitudes. In a polymeric bobbin basket, the incidence of fracture is dramatically reduced and the duration of useful life is greatly lengthened at lower operating speeds.
It is estimated that perhaps 60% of the world wide lock stitch sewing machines are rated to run above 4500 stitches per minute or 9000 RPM of loop taker rotation. These are considered high operating speeds. A commercially viable polymeric bobbin basket for use in a rotary loop taker must endure the stresses of an industrial sewing operation for about one year. For the reasons given above, it is necessary that the polymeric bobbin basket overcome its inherent weakness in the center post head and neck area in order to achieve side spread usage that realizes the full benefits of non-oiling, better stitch formation and lesser manufacturing costs.
Previous inventors have thought to invent rotary loop takers that outperform and are easier to manufacture than the conventional all steel model. They tried to achieve these objectives by using a synthetic moldable material with lower coefficients of friction. To date, there has not been significant market penetration of rotary loop takers using moldable bobbin baskets of polymeric materials.. It has been the inherent structural weakness of polymeric materials vis a vis metals that has prevented commercial viability of a bobbin basket formed of a synthetic moldable material. Polymeric bobbin baskets require unobvious and novel design to overcome the strength deficiencies relative to conventional metal parts. Violent needle impact and severe fatigue caused by vibration rendered the previous inventions unable to compete with the conventional metal parts in spite of the undeniable advantages of polymeric materials i.e., non-oiling, better stitch formation, less thread breakage.
Haas, U.S. Pat. No. 2,219,308, dated Oct. 29, 1940, developed an oscillating loop taker with a metal insert loop seizing beak to overcome needle deflection of his predominately polymeric shuttle hook. Although he understood that integrating a metal component with a primarily polymeric part would allow his low speed part to at least function, his approach teaches us nothing about how to overcome the structural deficiencies of a polymeric center post relative to its conventional metal counterpart. Haas' invention shows a complete circumferential undercut to define the center post neck, common to all conventional loop takers.
In Badillo's U.S. Pat. No. 4,858,543, dated Aug. 22, 1989, a bobbin basket is described using a metal needle deflection plate to shield the polymeric materials in an inherently vulnerable area was overcome by using metal to shield the weaker polymer below. Badillo clearly understood the necessity of designing around the inherent structural weakness of polymeric materials to solve the problem of severe needle deflection. With that invention, in fact, it became possible to use a polymeric bobbin basket in a mid speed lock stitch sewing machine without external lubrication for a limited length of time. However, Badillo did not realize at that time that the weakness in the center post neck and head portion would be another obstacle to achieving a commercially viable loop taker. Badillo's design shows again a complete circumferential undercut, common to all conventional loop takers. He teaches us nothing in that patent about strengthening or redesigning the center post area.
Herbst, in U.S. Pat. No. 3,016,033, dated Jan. 9, 1962 shows a polymeric oscillating loop taker with an integrally formed center post. It is not known to applicant whether this invention ever came to market. Like Haas, this invention was for a low speed oscillating loop taker and may have been able to endure the stresses and vibrations prevalent at those operating speeds; however it clearly could not withstand the high speeds of today's industrial equipment. Herbst's invention is molded with a complete circumferential undercut, to match exactly the conventional steel bobbin basket. Therefore it teaches nothing about solving the problem of structural weakness inherent in a center post constructed of a polymeric material.
The applicant is not aware of any successful attempts to market a polymeric bobbin basket for use in a rotary loop taker. The applicant discovered in his previous invention covered in U.S. Pat. No. 4,858,543, dated Aug. 22, 1989, the undeniable benefits in sewing performance and manufacturing that a polymeric bobbin basket can provide. The problem remains an inherent weakness in the fatigue strength of a polymer compared to a metal.